Egr-1 Expression in Smooth Muscle Cells
Smooth muscle cells (SMCs) are well recognized as a significant cellular component of atherosclerotic and post-angioplasty restenotic lesions (Stary et al, 1995; Holmes et al, 1984). SMC migration and proliferation are key events in the pathogenesis of these vascular disorders (Jackson & Schwartz, 1992; Libby et al, 1995). The promoter regions of many genes that encode mitogenic and migratory factors expressed by SMCs in these lesions (Evanko et al, 1998; Murry et al, 1996; Ueda et al, 1996; Tanizawa et al, 1996; Rekhter & Gordon, 1994; Hughes et al, 1993; Brogi et al, 1993; Wilcox et al 1989; Wilcox et al, 1988) contain nucleotide (nt) recognition elements for the nuclear protein and transcription factor, Egr-1 (Khachigian & Collins, 1997; Khachigian et al, 1996). Egr-1 is not expressed in the unmanipulated artery wall, but is rapidly activated by mechanical injury (Khachigian et al, 1996; Silverman et al, 1997; Kim et al, 1995). It is also induced in vascular endothelial cells and/or SMCs exposed to fluid biomechanical forces (Khachigian et al, 1997; Sumpio et al, 1998) and multiple other pathophysiologically-relevant agonists (Delbridge & Khachigian, 1997).
DNAzymes
In human gene therapy, antisense nucleic acid technology has been one of the major tools of choice to inactivate genes whose expression causes disease and is thus undesirable. The anti-sense approach employs a nucleic acid molecule that is complementary to, and thereby hybridizes with, an mRNA molecule encoding an undesirable gene. Such hybridization leads to the inhibition of gene expression.
Anti-sense technology suffers from certain drawbacks. Anti-sense hybridization results in the formation of a DNA/target mRNA heteroduplex. This heteroduplex serves as a substrate for RNAse H-mediated degradation of the target mRNA component. Here, the DNA anti-sense molecule serves in a passive manner, in that it merely facilitates the required cleavage by endogenous RNAse H enzyme. This dependence on RNAse H confers limitations on the design of anti-sense molecules regarding their chemistry and ability to form stable heteroduplexes with their target mRNA's. Anti-sense DNA molecules also suffer from problems associated with non-specific activity and, at higher concentrations, even toxicity.
As an alternative to anti-sense molecules, catalytic nucleic acid molecules have shown promise as therapeutic agents for suppressing gene expression, and are widely discussed in the literature (Haseloff (1988); Breaker (1994); Koizumi (1989); Otsuka; Kashani-Sabet (1992); Raillard (1996); and Carmi (1996)). Thus, unlike a conventional anti-sense molecule, a catalytic nucleic acid molecule functions by actually cleaving its target mRNA molecule instead of merely binding to it. Catalytic nucleic acid molecules can only cleave a target nucleic acid sequence if that target sequence meets certain minimum requirements. The target sequence must be complementary to the hybridizing regions of the catalytic nucleic acid, and the target must contain a specific sequence at the site of cleavage.
Catalytic RNA molecules (“ribozymes”) are well documented (Haseloff (1988); Symonds (1992); and Sun (1997)), and have been shown to be capable of cleaving both RNA (Haseloff (1988)) and DNA (Raillard (1996)) molecules. Indeed, the development of in vitro selection and evolution techniques has made it possible to obtain novel ribozymes against a known substrate, using either random variants of a known ribozyme or random-sequence RNA as a starting point (Pan (1992); Tsang (1994); and Breaker (1994)).
Ribozymes, however, are highly susceptible to enzymatic hydrolysis within the cells where they are intended to perform their function. This in turn limits their pharmaceutical applications.
Recently, a new class of catalytic molecules called “DNAzymes” was created (Breaker and Joyce (1995); Santoro (1997)). DNAzymes are single-stranded, and cleave both RNA (Breaker (1994); Santoro (1997)) and DNA (Carmi (1996)). A general model for the DNAzyme has been proposed, and is known as the “10-23” model. DNAzymes following the “10-23” model, also referred to simply as “10-23 DNAzymes”, have a catalytic domain of 15 deoxyribonucleotides, flanked by two substrate-recognition domains of seven to nine deoxyribonucleotides each. In vitro analyses show that this type of DNAzyme can effectively cleave its substrate RNA at purine:pyrimidine junctions under physiological conditions (Santoro (1997)).
DNAzymes show promise as therapeutic agents. However, DNAzyme success against a disease caused by the presence of a known mRNA molecule is not predictable. This unpredictability is due, in part, to two factors. First, certain mRNA secondary structures can impede a DNAzyme's ability to bind to and cleave its target mRNA. Second, the uptake of a DNAzyme by cells expressing the target mRNA may not be efficient enough to permit therapeutically meaningful results. For these reasons, merely knowing of a disease and its causative target mRNA sequence does not alone allow one to reasonably predict the therapeutic success of a DNAzyme against that target mRNA, absent an inventive step.